Cycles 'vector' pass meaning of W, X, Y, Z
Guided by @micaelh in the comments, I ended up doing some experiments moving objects in a controlled way, the results are below.

Vector.X - X displacement from previous frame to current frame
Vector.Y - Y displacement from previous frame to current frame
Vector.Z - X displacement from current frame to next frame
Vector.W - Y displacement from current frame to next frame

Values are in pixels. X displacement is positive to the left, Y displacement is positive downwards.